For me it depends on what you mean by "earthlike". If i want something very much like Earth, a terra with multicellular life on land, around an K, G or F main sequence star, and not being tidally locked to said star. If i want something not that earthlike, i just search for a terra, and for in-between, somewhere between the previous two scenarios.

And i often use atmosphere restrictions and disable tidally locked planets, and occasionally use other restrictions.

I've been searching for Earthlike planets for the last few days but every single one, bar one or two, out of dozens has a very high CO2-content atmosphere. It is a little bit of a stumbling block. Why is this, is it realistic or the way the game engine has been designed?

I would like to think our future explorers can terraform such nearly-suitable exoplanets, but that would kill out all the life already there, presumably

Source of the post Something to do with lots of vegetation and photosynthesis and very little aerobic respiration, perhaps?

Photosynthesis produces O2 not CO2 (6 CO2 + 6 H2O + light = C6H12O6 + O2). But of course these aren't earth plants, and maybe use something other than photosynthesis. I don't think the atmospheric composition in 0.9.8.0 is realistic (you have to remember that atmospheric composition was added in the current update), but my best guesses is that either they have more life that uses O2 like animals instead of CO2 like plants, or that there isn't as many plants.

These are for all sorts of planets about which I think may be tarraformable. There are plenty of such. The lower mass limit of 0.05 is just under the mass of Mercury, which has nearly the same surface gravity as Mars. And we think about terraforming Mars a lot. I set the upper limit of 3.5 Earth masses... "by eye" because of high surface gravity. Any more than that and it would be above 1.7 or 1.8 G, which would not be pleasant.

These parameters are for planets which could already support humans. 0.4 atmospheric pressure would suffice, if such atmosphere was oxygen rich. 3 atmospheres may be a lot for an earthlike atmosphere, but it would drop quickly with altitude. Pressure at Mount Everest is at 30% of the sea level pressure. Aurora marked as required? ..Magnetic field is important for an array of reasons. 0.05 mass is very low for this searching. The lightest terra I found was at 0.109 Earth masses.

Other than that... If I am looking for an Earth twin, I specify as much as the game allows

Photosynthesis produces O2 not CO2 (6 CO2 + 6 H2O + light = C6H12O6 + O2). But of course these aren't earth plants, and maybe use something other than photosynthesis. I don't think the atmospheric composition in 0.9.8.0 is realistic (you have to remember that atmospheric composition was added in the current update), but my best guesses is that either they have more life that uses O2 like animals instead of CO2 like plants, or that there isn't as many plants.

Ah now that makes more sense... Yes, I found a few planets recently with high levels of CO2 which I thought could potentially be terraformed if trees were planted on them... is that a valid way of terraforming?

And if the atmospheric CO2 levels aren't toxic, the SO2 almost always is

Yes, have found a few deceptive planets which check out in all regards, until I notice the deadly levels of Sulfur Dioxide or sometimes methane.